This invention relates to the processing of titanium articles fabricated by powder metallurgy of prealloyed titanium powder to improve the microstructure of such articles.
In general terms, powder metallurgy involves production, processing and consolidation of fine particles to produce a solid article. The small, homogeneous powder particles result in a uniform microstructure in the final product. If the final product is made net-shape by application of hot isostatic pressing (HIP), a lack of texture can result, thus giving equal properties in all directions.
Titanium powder metallurgy is generally divided into two "approaches", the "elemental approach" and the "prealloyed approach". With the "elemental approach", the small (-100 mesh) regular grains of titanium normally rejected during the conversion of ore to ingot (commonly called "sponge fines"), are used as starting stock. Alloy additions, normally in the form of a powdered master alloy, are added to these fines, so that the desired bulk chemistry is achieved. The blended mixture is then compacted cold, under pressures up to 420 MPa (60 ksi), to a density of 85-90%. This operation can be carried out either isostatically or with a relatively simple mechanical press. The "green" compact is then sintered to increase density to 95-99.8% theoretical density and to homogenize the chemistry. The cold isostatic pressing is often referred to as CIP. A further increase in density can be achieved by hot isostatic pressing the article, which also generally improves the mechanical properties of the article. The combined cold/hot isostatic pressing process is referred to as CHIP.
The CHIP process using elastomeric molds can produce extremely complex shares, which are very difficult to achieve by forging processes. Caution must be used in applying parts made by this technique in critical components, such as rotating parts, where fatigue behavior is usually very important. Available data indicate that parts made from elemental material are inferior to wrought material in fatigue performance.
With the "pre-alloyed approach", spherical pre-alloyed powder is used. Spherical powder flows readily, with minimal bridging tendency, and packs to a very consistent density (approximately 65%). This leads to excellent part-to-part dimensional reproducibility. Pre-alloyed powder is generally HIP'd or otherwise hot pressed. Parts made from pre-alloyed powder generally exhibit better fatigue performance than those made of elemental powder, but are somewhat inferior to wrought material.
While mechanical working can be used on wrought articles to modify their structures and enhance properties, such treatment is not practical for net shape articles produced from powder. Eylon et al. describe in U.S. Pat. Nos. 4,534,808 and 4,536,234, methods for refining the microstructure of articles made by powder metallurgy which generally comprise the steps of beta-solution heat treating the article for a relatively brief time, quenching the article, aging the article at a suitable temperature and air cooling the article to room temperature. These methods provide articles having a fine alpha plate microstructure in a matrix of a discontinuous beta phase, which exhibit increased fatigue strength.
We have found that the microstructure of net shape prealloyed titanium powder compacts can be further improved.
Accordingly it is an object of the present invention to provide a process for improving the microstructure of titanium articles made by powder metallurgy of prealloyed titanium powder.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art after reading the detailed description of the invention as well as the appended claims.